Last year, online retail buzz centered on razors. Dollar Shave Club, No. 238 in the Internet Retailer Top 500 Guide, and Harry’s Grooming (No. 493) were growing at a record pace by using aggressive and sometimes quirky marketing tactics to promote their monthly subscription services that sends a consumer razor blades for monthly fees of $3, $6 or $9. They each raised more than $100 million in venture capital funding, and even threatened long-time, established razor blade brands.  Proctor & Gamble-owned Gillette launched its  own version of the online subscription service as a result.

This year? The craze is mattresses.

Three of the top six fastest-growing e-retailers in the Top 500 (Leesa, ranked No. 499, Saatva Inc. at No. 300, and Casper at No. 290) sell their own brand of mattresses—a product consumers mostly have purchased in the past at bricks-and-mortar mattress stores and from a few online players that previously captured only modest market share. But several retailers that sell mattresses online have popped up in the last couple of years, many raising millions in venture capital funding and growing fast. These three merchants are new to the Top 500 rankings this year, and they each skipped The Second 500 Guide (Internet Retailer’s ranking of the fast-growing mid-tier in online retail) entirely, a sign of their rapid emergence from startup status to meaningful sales.

Take Casper, a foam mattress maker that drove an estimated $75 million in online sales last year—its first full year in business. Casper sells only one type of mattress, which costs $500 to $950 depending on the size, and it folds its mattresses up to fit in a large Casper-branded box for shipment to consumer through traditional carriers. The merchant runs an attractive program that gives shoppers $50  for any referrals that end in a sale, and its unique marketing tactics have succeeded in spreading the word about the comfort and convenience of its products. For example, the merchant owns a Casper Sleep-branded semi-truck that outfitted with Casper mattresses and sleeping pods. Staffers drive the vehicle around urban areas and offer free nap sessions to potential customers.

Casper co-founder Luke Sherwin says the retailer is taking advantage of the reach of the Internet to transform the inefficient way merchants have sold mattresses in the past. “The idea of owning millions of square feet of store space while trying to sell 10 mattresses is something that perpetuated for 25 years with little change,” Sherwin says. “The majority of consumers just want something different, and because we’re giving them a trial period of 100 nights to sleep on our mattresses, we’re removing any tension that exists in wanting to try it out in a store first. We’ve been able to capture and recalibrate that whole paradigm and literally just provide a new way to buy a mattress.”

Saatva, which has been around the longest of the three, aims to stand out from its competitors by providing luxury customer service, says its co-founder and chief marketing officer Ricky Joshi. Saatva, for example, has its own delivery network and for $99 a trained professional will deliver and set up a customer’s mattress. For an extra $39, the merchant will take away an old mattress. This service contrasts with Casper Sleep and Leesa, whose foam mattresses fold up to fit in a large box and are shipped through traditional carriers.

All three merchants’ queen mattress sells for under $900. Saatva’s queen sells for $899 not including delivery. Casper’s queen mattress costs $850 with free shipping. A Leesa queen runs $890 with free shipping.

Profiles of more fast-growing merchants are included in the new Internet Retailer Top 500 Guide. Click here for more information, or to order a copy.